The present invention generally relates to line switching apparatuses, and more particularly to a line switching apparatus for use with a communication apparatus which accommodates a plurality of lines according to a key number system and a line switching apparatus which is coupled between a communication apparatus which accommodates a plurality of lines and a telephone line.
In communication apparatuses such as a facsimile store and forward apparatus and an answering apparatus which accommodate a plurality of lines, the lines are often accommodated according to a key number system. According to the key number system, the plurality of lines which are connected to the communication apparatus via the line switching apparatus are supervised by key numbers. When the line switching apparatus receives a call in to a subscriber of the communication apparatus, a line busy detection is made from a first key number. When the line of the first key number is busy, the line busy detection is made with respect to a second key number. The line busy detection is made until a non-busy line is detected or until it is detected that all of the lines are busy when carrying out a line detection process.
The communication apparatus has a plurality of ports, and a communication terminal having a communication function is coupled to each port. For example, the communication terminal may be a facsimile terminal, a talkie apparatus and the like. Conventionally, the connection of the lines and the ports of the communication apparatus is fixed, and the lines and the ports are directly connected, for example.
Accordingly, when the communication terminal which is coupled to a port is making a communication, the line which is coupled to this port is busy. Hence, when the line switching apparatus receives a call in to the subscriber, the line detection process is carried out to call a non-busy line.
On the other hand, there is a communication apparatus which accommodates a plurality of lines in one-to-one correspondence with a plurality of ports thereof, without employing the key number system. In this case, when the communication terminal which is coupled to a port is making a communication, the line which is coupled to this port is busy until the communication terminal ends the communication.
But because the conventional line switching apparatus has a fixed connection between the lines and the ports, a problem occurs when a port is down. The port is said to be "down" when the port is not capable of making a communication. When the port is down during the line detection process, the line which is coupled to the port which is down is called continuously and an appropriate line detection process cannot be made.
When the key number system is employed, the line switching apparatus carries out the line detection process in which the call is made from the line of the first key number to detect whether or not the line is busy and the call is made to the line of the next key number when the line of the first key number is busy. However, when the call is made to the port which is down, the line switching apparatus continues to call the same line because no response is received and no call is made to the line of the next key number. As a result, the line detection process stops at the port which is down, and it is impossible to call the line of the next key number.
On the other hand, in the case of the communication apparatus which accommodates the lines in one-to-one correspondence with the ports, no response is received from the port which is down until the port is restored. Therefore, the service deteriorates for the user who calls the communication apparatus from the communication terminal.